A source of benzene and xylenes is catalytic reformate, which is prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C6 to C8 fraction is separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains BTX, along with ethylbenzene.
Refineries have also focused on the production of benzene and xylenes by transalkylation of C9+ aromatics and toluene over noble metal-containing zeolite catalysts. High value petrochemical products, such as benzene and xylenes, together with ethylbenzene are formed during the transalkylation of C9+ aromatics and toluene over catalysts containing noble metals. The resulting translation product is subjected to further separation of non-aromatics, benzene, C8 aromatics (i.e., ethylbenzene, para-xylene, meta-xylene, and ortho-xylene), unreacted toluene, and unreacted C9+ aromatics. Usually, the C8 aromatics product is subjected to further separation to produce xylenes, particularly, para-xylene. Lowering the amount of ethylbenzene in the C8 aromatics improves efficiency of xylene recovery. Therefore, there are strong economic and technical incentives to decrease the ethylbenzene concentration in the transalkylation product. The amount of ethylbenzene in the transalkylation product depends primarily on (a) the feedstock composition and (b) the transalkylation catalyst and the transalkylation conditions. Typically, the C9+ aromatics feedstock and/or the toluene feed contains ethylbenzene as an impurity ranging from 0.001 to 4 wt % based on total weight of the feed. Other than the ethylbenzene in the feedstock, ethylbenzene can be formed during transalkylation process from a feed comprising ethyl-containing C9+ aromatics and from various side-reactions.
One solution to the problem of the ethylbenzene in the transalkylation product during the transalkylation of heavy aromatics is disclosed in U.S. Pat. No. 5,942,651 and involves the steps of contacting a feed comprising C9+ aromatic hydrocarbons and toluene under transalkylation reaction conditions with a first catalyst composition comprising a zeolite having a constraint index ranging from 0.5 to 3, such as ZSM-12, and a hydrogenation component. The effluent resulting from the first contacting step is then contacted with a second catalyst composition which comprises a zeolite having a constraint index ranging from 3 to 12, such as ZSM-5, and which may be in a separate bed or a separate reactor from the first catalyst composition to produce a transalkylation reaction product comprising benzene and xylene. The ethylbenzene in the feed and/or the ethylbenzene formed during transalkylation process is partially destroyed by dealkylation of ethylbenzene to benzene and ethylene.
U.S. Pat. No. 5,905,051 discloses a process for converting a hydrocarbon stream such as, for example, a C9+ aromatic compound to C6 to C8 aromatic hydrocarbons, such as xylenes, by contacting the stream with a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein said catalyst compositions are present in separate stages and are not physically mixed or blended and wherein said first catalyst composition is a metal-promoted, alumina- or silica-bound zeolite beta, and said second catalyst composition is ZSM-5 having incorporated therein an activity promoter selected from the group consisting of silicon, phosphorus, sulfur, and combinations thereof. According to the '051 patent, the use of the separate catalytic stages improves the conversion of C9+ aromatic compounds and naphthalenes to xylenes and decreases the amount of undesirable ethylbenzene in the product. The ethylbenzene in the '051 product is about 3-7 wt % of ethylbenzene based on the weight of C8 aromatics fraction of the resulting product.
It has now been found that a catalyst system comprising a molecular sieve exhibits enhanced acidity and hydrogenation activity for substantial removal of ethyl-group containing aromatic compounds in C9+ aromatic feeds without overall reduction in the conversion of the C9+ feed to useful compounds, such as xylenes.